Extensive systems have been deployed to use GPS capabilities for the purpose of tracking vehicle fleets of, e.g., truck trailers, truck tractors and/or trucks; or railcars. Such systems have been referred to as “asset tracking systems” and deploy asset tracking units designed to be attached to individual vehicles. Each asset tracking unit includes a GPS receiver that is capable of receiving GPS signals from a plurality of GPS satellites and determining the unit's location based on the GPS signals. Typically simultaneous or nearly simultaneous receipt of signals from at least three GPS satellites (to provide a 2D fix without altitude) or four GPS satellites (to provide a 3D fix with altitude) is required to fix the unit's location. When a fix is obtained, the asset tracking unit may report the unit's location via satellite communication (using another set of satellites) or the like to a central station. With such a system, the proprietor of the vehicle fleet will have information concerning the whereabouts of all vehicles in the fleet. This may lead to significant efficiencies in planning and managing assignments of vehicles to particular tasks. In addition, an asset tracking system of this type may help in the detection of, and response to, irregularities such as theft of vehicles.
Asset tracking units may operate on battery power, and may be required to operate in the field for extensive periods of time without recharging or replacement of batteries. Accordingly, power management may be a critical issue for satisfactory deployment and use of GPS-based asset tracking units. The activity of the GPS receiver component of the asset tracking unit may represent a significant portion of the power drain on the unit battery. It is therefore highly desirable that an asset tracking unit be operated in a manner that maximizes the likelihood of successful acquisition of the requisite number of GPS satellite signals, while minimizing power consumption.